Introduce ftl::ignore

`ftl::ignore` works like `std::ignore`, but one can also ignore multiple
values using `operator()`. `ftl::ignore(a, b);` is more compact than
`std::ignore = a; std::ignore = b;`, and conveys the same idea.

Bug: 185536303
Flag: EXEMPT New library code
Test: atest ftl_test

Change-Id: Ice549328eeffb90f59f7b902809def48f583d157